Obstacles and catalysts of cooperation in humans, bonobos, and chimpanzees: behavioural reaction norms can help explain variation in sex roles, inequality, war and peace

Abstract

Our closest living relatives, bonobos and chimpanzees, along with small-scale human societies figure prominently in debates about human nature. Here we emphasize and explain behavioural variation within and among these three species. In the logic of behavioural ecology, individuals have been selected to adjust their behaviour along evolved reaction norms that maximize fitness given current socio-ecological conditions. We discuss variation in three behavioural contexts: relationships between the sexes, hierarchy and inequality, and intergroup interactions. In each context, behavioural variation can be related to two broad socio-ecological conditions: (i) the defensibility of contested resources, and (ii) differences in bargaining power. When defensibility of resources and differences in bargaining power are great, interactions are rife with conflict; when they are minimal, interactions are more harmonious. These socio-ecological conditions therefore constitute key catalysts and obstacles of cooperation. We conclude that human nature should be seen as consisting of evolved reaction norms.

Men’s status and reproductive success in 33 nonindustrial societies: Effects of subsistence, marriage system, and reproductive strategy

“…Observation and archaeology of foragers reveals tremendous variation in status hierarchy (25). In low-density relatively nomadic forager societies, decisionmaking is typically consensus based (at least among adult men), and status inequality is limited by fluid group membership, coalitional checks on would-be dominants, and cooperative production and interdependence (14,(25)(26)(27). Leadership tends to arise occasionally to meet situational demands and typically involves little or no material benefit relative to followers (28,29).…”

Observations of severe and lethal coalitionary attacks in wild mountain gorillas

“…It is therefore unsurprising that the vertebrate animal behavior corollary best approximating human warfare occurs in one of humans’ closest extant relative, chimpanzees23. As is true for humans, rates of such encounters vary widely across sites and social groups4, but lethal chimpanzee ‘raids’ and other forms of cooperative intergroup attacks have been regularly reported at multiple long-term field sites2567891011.…”

“…Between and within species, violent intergroup conflict is most likely to occur when important resources are defensible and demographic power imbalances reduce the cost to individual participants411121314. These socioecological conditions help explain differences in relative rates of both human and chimpanzee ‘warfare,’ across sites and groups, as well as its near-absence in other close relatives including bonobos234 and orangutans15.…”

“…These socioecological conditions help explain differences in relative rates of both human and chimpanzee ‘warfare,’ across sites and groups, as well as its near-absence in other close relatives including bonobos234 and orangutans15. Ultimate explanations for the evolution of coalitionary aggression include direct benefits via improved resource access11121314 or status maintenance/elevation1617, or indirect benefits via kin selection and reciprocal altruism1819.…”

“…First, the modal group type is one male with multiple females and their offspring, which limits opportunities for coalitions26. Second, resource defense is often considered an important motivator of such behavior in great apes3411, but unlike chimpanzees and (historically) humans, mountain gorillas are herbivores with an abundant year-round food supply2728. Far less is known about the behavior of more frugivorous western lowland gorillas ( G. gorilla ), but since they also primarily occur in single-male groups opportunities for male coalitions are limited, and benefits of female coalitions are probably minimal29.…”